The present invention relates to a novel method of producing 3-bromobenzaldehyde. 3-Bromobenzaldehyde obtainable from the present invention is an useful substance as a raw material for the insecticides of synthesized pyrethroid type.
As a conventional method, there is one described in Japanese Unexamined Patent Publication No. Sho 51-143639. According to this disclosure, benzaldehyde is allowed to react with bromine at a temperature below about 100.degree. C. in the presence of chlorinated hydrocarbon solvent and brominating catalyst and the amount of brominating catalyst used is at least 1 mole per mole of the benzaldehyde.
In this method, 3-chlorobenzaldehyde, which is a by-product, is formed partially because of the use of chlorine and yet the starting material is left behind in amounts not less than 10%. Thus, this method is hard to say to be efficient.
For this reason, the theme of the invention lies in finding a method to produce 3-bromobenzaldehyde by improvement of the shortcomings of the methods known in public, which are partial formation of 3-chlorobenzaldehyde being a by-product and remaining considerable amounts of unreacted benzaldehyde.
As a result of diligent studies aiming at the development of an excellent method for the synthesis of 3-bromobenzaldehyde, the inventors have found that, upon reacting bromine with benzaldehyde in 1,2-dichloroethane being a reaction solvent in the presence of brominating catalyst, 1,2-dichloroethane changes partially into 1-bromo-2-chloroethane and 1,2-dibromoethane and bromine used is not released out of the system at all. Thus, this solvent recovered was allowed to react with chlorine in the presence of brominating catalyst, it was surprisingly found that 1-bromo-2-chloroethane and 1,2-dibromoethane were regenerated to 1,2-dichloroethane and yet bromine was generated. Based on this discovery, a method of producing 3-bromobenzaldehyde has been completed, leading to the present invention.